class Parent
  include Mongoid::Document

  has_and_belongs_to_many :childs, :inverse_of => nil, :validate => false
end

class Child
  include Mongoid::Document
  
  field :hi, :type => String
end

nader$ rails c
Loading development environment (Rails 3.1.1)
ruby-1.9.2-p290 :001 > p1 = Parent.create
 => #<Parent _id: 4f7348308035440918000001, _type: nil, child_ids: []> 
ruby-1.9.2-p290 :002 > p1.childs
 => [] 
ruby-1.9.2-p290 :003 > p1.child_ids = [Child.first.id, Child.last.id]
 => [BSON::ObjectId('4f7346d980354408da000001'), BSON::ObjectId('4f73472380354408da000004')] 
ruby-1.9.2-p290 :004 > p1.save
 => true 
ruby-1.9.2-p290 :005 > p1.childs
 => [] 
ruby-1.9.2-p290 :006 > p1.childs.count
 => 2 
ruby-1.9.2-p290 :007 > p1.childs.length
 => 0